Electrostatic printing system utilizing a. c. and d. c. bias



April 19, 1966 J. J. STONE, JR 3,

ELECTROSTATIC PRINTING SYSTEM UTILIZING AND D.C. BIAS Filed Oct. 1, 1962 ,5 lNTE/VS/TY 2 CONT/P01.

SIGNAL SOURCE DEFLECT/O/V FIG I s/a/m 47 50mm L l2 /3 \j M 46 s/a/v/u llg SOURCE FIG. 3

FIG 4 INVENTOR. Joseph James 5/0/76 Jr BY 0 M m am Maw @YM A1775.

United States Patent 3,247,517 ELECTROSTATIC PRINTING SYSTEM UTILIZING A.C. AND D.C. BIAS Joseph James Stone, Jr., Glenview, Ill., assignor to A. B.

' Dick Company, Niles, Ill., a corporation of Illinois Filed Oct. 1, 1962, Ser. No. 227,500 3 Claims. (CL. 346--74) This invention relates in general to an improved electrostatic printing arrangement, and more particularly to an arrangement for improving the linearity and resolution of images secured by means of electrostatic printing tubes and for enabling the use of lower level writing voltages.

This is a continuation-in-part of my copending application Ser. No. 14,689, filed March 14, 1960, now abandoned, and entitled Electrostatic Printing System.

In known electrostatic printing arrangements such as that disclosed in an application by John S. Tregay, Jr., filed May 10, 1957, Ser. No. 658,275, now abandoned, a conventional cathode ray tube is modified by the provi- 3,247,517 Patented Apr. 19, 1966 rangement for modulating the potential between the sion of an array of wires in its face, which extend from the interior surface of the face to the exterior surface. A high voltage beam of electrons is varied in accordance with a desired image and traverses the interior terminations of the array of wires embedded in the non-conductive face of the tube to cause a discharge through a dielectric or printing paper passing adjacent to the exterior wire terminations at a desired speed. This arrangement, although producing satisfactory images, does give rise to some difficulties in resolution due to the capacitativeeffect of the wires in the matrix.

The beam is operated at high potentials, for example, l0 K.V., while the signals representing the object to be reproduced modulate the beam at much lower voltages and at comparatively slow frequencies. This arrangement often produces poor resolution and insufficient image linearity.

In addition, thehighbeamcurrents give rise to con: siderable secondary emission and a host-of interrelated electricalconditions all of which contributes to the problems of securing good reliable image resolution.

It is, therefore, proposed by the present invention'considerably to increase the effectiveness of the signal used for modulating thebeam and to do so in a simple, effective and economical manner.

This is done by, providingan alternating high frequency signal at writing potential onto the dielectric printing paper and superimposing the signal beam from the cathode ray tube thereon whereby a signal modulated beam at considerably lower voltage can be employed to inscribe the desired electrostatic image on the dielectric printing paper and thus achieving the desired improvements in image resolution and linearity.

Accordingly, it is an'object of this invention to provide an electrostatic printing system characterized by image resolution and linearity.

Another object of this invention is to provide an improved system for modulating the voltage difference between the writing wires used in an electrostatic printing tube and the dielectric recording medium to lower the necessary beam modulating voltage for producing a writing discharge.

A feature of this invention resides in the arrangement for superimposing a video and an alternating high frequency signal at writing potential to modulate the voltage difference between the writing wires and recording medium whereby a video signal at a voltage insuflicient to reach writing potential becomes effective to cause a resultant electrostatic image to be retained on the dielectric writing medium.

Another feature of this invention resides in the ar- Writing wires and the recording medium with an alternating high frequency signal of sufiicient magnitude to reach writing potential without recordingthe alternating high frequency signal and enabling the use of lower beam energies in the video signal from the cathode ray tube to produce a residue of the desired image on the dielectric writing medium.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of the invention is shown in the accompanying drawing, in which FIG. 1 is a standard cathode ray tube of the type used in electrostatic printing embodying the application of an alternating high frequency signal on which the video signal is superimposed in accordance with the practice of this invention;

FIG. 2 represents the wave form of the alternating high frequency signal;

FIG. 3 represents the wave form of the image signal from the cathode ray tube; and

FIG. 4 shows the wave form of the composite signal that is formed by superimposing the image signal on the alternating high frequency signal.

Referring now to FIG. 1, showing a cross-section of the electrostatic printing tube embodying the features of this invention, it will be seen that the tube indicated at it) is similar to a conventional cathode ray tube. It includes an evacuated envelope 12, which carries a cathode 14, a control. .grid 13 connected to a suitable signal source 15, an anode or accelerating electrode 16, a focus coil 18,- and a deflection coil 20. The cathode is connected to the negative terminal or high voltage supply 22. p In addition; the tube includes a face plate 32 of dielectric materialin which anarray of wires 34 are carried or embedded-to extend through the face of the tube from the inside to the. outside. Adjacent the terminations of the wires, and exterior of the tube, a suitable dielectric material .36 is provided for recording the image. The

- dielectric material is moved in the direction indicated by the arrow and has associated on its other or back side a conductor 38, at ground potential whereby discharge from the wires through the dielectric can be accomplished.

The deflection coil 20 is controlled from a suitable deflection signal source 28 for controlling the beam produced at the cathode 14, while the signal source 15 introducesthe image signals representing the object to be, reproduced at the control grid. Such signal is suppliedj in the formof negative voltage, as illustrated by the wave form in FIG. 3.

Also connected to the dielectric material 36 is a source of alternating high frequency 45. This source 45 is operated at high frequency, such as megacycles, and at sufficiently high voltage at the peak signal amplitudes to reach writing potential, such as 500 volts, as illustrated by the alternating wave form of FIG. 2. The resistors 46 and 47 operate electrically to isolate the sources 15 and 45. The resultant signal produced by superimposing the signal from signal source 45 and from source 15 is shown in FIG. 4 and the resultant signal is applied across the gap between the wires 34 and the dielectric 36.

The theory believed to underlie this arrangement can be set forth as follows: At the peak amplitudes of the high frequency alternating current introduced from the signal source 45, an electrostatic image is inscribed on the dielectric printing medium. However, the positive charge that is deposited when the alternating current is on the plus side is erased by the negative charge that is deposited by the alternating current of the same amplitude when on the negative side so that no electric charge image is left on the dielectric printing medium when only the high frequency alternating current is applied from the source 45. The erasure of the positive by the negative results from the very narrow lapse of time between the change from positive to negative in the alternating high frequency current. Nevertheless, the peak amplitude or voltage of the alternating high frequency current is sufficiently high to cause the deposition of an electrostatic image in the event that the wave form is disturbed or displaced whereby the one charge is not completely wiped out by the opposite charge. This unbalance in wave form is achieved by the superimposing of the video signal which can thus be of low voltage since the video signal itself need no longer be relied upon for amplitudes high enough to reach writing potential or to discharge and transcribe the electrostatic image on the dielectric writing medium.

The video signal, at low potential, from the signal source 15, is in the form of a negative potential. Thus the negative potential from the signal source 15 operates to increase the differential between the wires and dielectric writing material when the peak amplitude of the alternating signal source 45 is positive and decrease the differential when the peak amplitude of the alternating signal source 45 is negative. As a result, the discharge at positive potential is far greater than the discharge at negative potential when the video signal is imposed onto the high frequency alternating current with the result that an electrostatic discharge corresponding to the image remains on the dielectric writing medium for subsequent development or use.

In operation, the primary signal beam is, of course, directed to the various wires 34, in accordance with the desired scanning rate, while the image reproducing dielectric 36 passes adjacent the external terminations of the wires.

The control grid 13 is controlled in accordance with the signals introduced thereto from signal source 15 and modulates the beam emitted by 14 accordingly to charge the Wires 34. It will be noted from the wave form shown in FIG. 4 that the changes in amplitude for various portions of the composite signal between the wires and dielectric 36 are far more marked than amplitude changes shown in FIG. 3 and they occur in great frequency. The potential source 22 furnishes a voltage of sufiicient magnitude to provide a primary beam which need not be operated at the high voltages previously used, for the reasons previously pointed out.

Thus each Wire 34 is energized in sequence and repetitively as the beam sweeps across the face at a rate determined by the deflection signal source 28. The degree of charge imparted to each wire 34 is dependent on the modulation of the beam, but as the beam traverses the wires at a rate corresponding to the image signal frequency, the sampling effect produced by source 45 is averaged to cause the wires to discharge through dielectric 36 in accordance with the input from 15. With the described arrangement it has been found possible to produce images'of both good resolutions and linearity and at comparatively low beam currents under conditions under which the conventional electrostatic tube has not produced any images, and although the applicant is not fully cognizant of the reasons therefor, the above has been proposed by way of explanation.

In accordance with the above, there has been shown and described herein a novel, useful and simple arrangement for improving the operation of an electrostatic writing tube, but the particular embodiments or forms of the invention described herein are not limitations upon other manners of practicing the invention.

I claim:

1. In an electrostatic copy system, the combination comprising a cathode ray tube having a cathode and anode elements for producing an electron beam, a direct current power supply having a negative voltage terminal connected to said cathode and positive voltage terminals connected to said anode elements, said cathode ray tube having a face with a plurality of closely spaced conductive elements extending therethrough, said conductive elements having inner ends inside said tube and outer ends outside said tube, means for deflecting said electron beam to produce scanning of the inner ends of said conductive elements by said electron beam, means including a guiding conductor for carrying a thin dielectric writing medium past the outer ends of said conductive elements, said guiding conductor being closely spaced from the outer ends of said conductive elements, said cathode ray tube having a control element for controlling the electron beam, means for supplying a video signal to said control element to modulate the electron beam in accordance with said video signal, D.C. biasing means for supplying a positively polarized direct voltage in excess of the direct voltages applied to said anode elements of said tube, A.C. biasing means for supplying high frequency alternating current at a high voltage, and means for connecting said D.C. biasing means and said A.C. biasing means in series to said guiding conductor to provide combined DC. and A.C. biasing voltages thereto so as to assist in the production of writing discharges in said dielectric writing medium.

2. The combination of claim 1, in which said high frequency alternating voltage has a peak amplitude which reaches writing potential sutficient to produce writing discharges in the dielectric writing medium.

3. The combination of claim 1, in which the frequency of said high frequency alternating voltage is of a higher order of magnitude than the frequencies of said video signals. Y

References Qited by the Examiner UNITED STATES PATENTS 3,040,124 6/1962 Cami-as 34674 IRVING L. SRAGOW, Primary Examiner. 

1. IN AN ELECTROSTATIC COPY SYSTEM, THE COMBINATION COMPRISING A CATHODE RAY TUBE HAVING A CATHODE AND ANODE ELEMENTS FOR PRODUCING AN ELECTRON BEAM, A DIRECT CURRENT POWER SUPPLY HAVING A NEGATIVE VOLTAGE TERMINAL CONNECTED TO SAID CATHODE AND POSITIVE VOLTAGE TERMINALS CONNECTED TO SAID ANODE ELEMENTS, SAID CATHODE RAY TUBE HAVING A FACE WITH A PLURALITY OF CLOSELY SPACED CONDUCTIVE ELEMENTS EXTENDING THERETHROUGH, SAID CONDUCTIVE ELEMENTS HAVING INNER ENDS INSIDE SAID TUBE AND OUTER ENDS OUTSIDE SAID TUBE, MEANS FOR DEFLECTING SAID ELECTRON BEAM TO PRODUCE SCANNING OF THE INNER ENDS OF SAID CONDUCTIVE ELEMENTS BY SAID ELECTRON BEAM, MEANS INCLUDING A GUIDING CONDUCTOR FOR CARRYING A THIN DIELECTRIC WRITING MEDIUM PAST THE OUTER ENDS OF SAID CONDUCTIVE ELEMENTS, SAID GUIDING CONDUCTOR BEING CLOSELY SPACED FROM THE OUTER ENDS OF SAID CONDUCTIVE ELEMENTS, SAID CATHODE RAY TUBE HAVING A CONTROL ELEMENT FOR CONTROLLING THE ELECTRON BEAM, MEANS FOR SUPPLYING A VIDEO SIGNAL TO SAID CONTROL ELEMENT TO MODULATE THE ELECTRON BEAM IN ACCORDANCE WITH SAID VIDEO SIGNAL, D.C. BIASING MEANS FOR SUPPLYING A POSITIVELY POLARIZED DIRECT VOLTAGE IN EXCESS OF THE DIRECT VOLTAGES APPLIED TO SAID ANODE ELEMENTS OF SAID TUBE, A.C. BIASING MEANS FOR SUPPLYING HIGH FREQUENCY ALTERNATING CURRENT AT A HIGH VOLTAGE, AND MEANS FOR CONNECTING SAID D.C. BIASING MEANS AND SAID A.C. BIASING MEANS IN SERIES TO SAID GUIDING CONDUCTOR TO PROVIDE COMBINED D.C. AND A.C. BIASING VOLTAGES THERETO SO AS TO ASSIST IN THE PRODUCTION OF WRITING DISCHARGES IN SAID DIELECTRIC WRITING MEDIUM. 